Thermal imaging systems have achieved an important status in military and commercial operations. These systems do not require any supplemental radiation other than that radiated by the object or scene under investigation. The radiation involved is in the infrared region. Current real-time systems rely primarily on semiconductor diodes as the detecting media. These in turn depend on certain bandgap energies which limit their efficiency to rather narrow bands. Most systems employ a linear array of diodes across which the image is swept by a scanning mirror. Additionally, the diodes work well only at cryogenic temperatures, and such systems are noisy, bulky and inefficient.
Retinas consisting of bolometric devices have been used successfully with electron scanning, but these lack resolution in the infrared region. These retinas also require cryogenic cooling, which is made more difficult by the hard vacuum requirements of the system.
A new retina has been introduced in fairly recent times in a device known as the pyroelectric vidicon. This retina utilizes changes in the electric polarization induced in a pyroelectric material when exposed to radiation. Since the retina is only responsive to temperature changes in the scene projected on it, it makes an excellent moving target indicator. Fixed targets or scenes can be viewed in a variety of operational modes such as by panning the camera, or otherwise modulating the scene intensity. Since the retina works well at room temperature, power requirements are modest. The chief difficulty with these devices has been that the retinas are subject to erosion by the electron beam in the vidicon and are extremely sensitive to moisture. As a result vidicons made from them, though initially testing out satisfactorily, have been usually very short lived, i.e. at most a few hundred hours.